The nucleosome, the basic unit of DNA packaging in eukaryotes that consists of a 147-bp DNA wound in sequence around a histone protein core, is a fundamental unit of chromatin structures [Non-Patent Document 1]. All four core histones (H3, H4, H2A and H2B) possess unstructured N-terminal tails and these N-termini of histones are particularly subjected to a diverse array of post-translational modifications: acetylation, methylation, phosphorylation, ubiquitination, SUMOylation and ADP-ribosylation [Non-Patent Document 2]. These histone modifications cause dynamic changes to the chromatin structure and thereby impinge on transcriptional regulation, DNA replication, DNA repair, and alternative splicing [Non-Patent Documents 3 and 4]. Among these epigenetic marks on histones, the methylation process is particularly crucial for transcriptional regulation [Non-Patent Document 5]. Five lysine residues (H3K4, H3K9, H3K27, H3K36 and H4K20) are located in the N-terminal tails and are representative lysines that can become mono-, di-, or trimethylated. Whereas H3K9, H3K27 and H4K20 methylation mainly represses transcription, methylation marks on H3K4 and H3K36 are associated with the induction of active transcription [Non-Patent Document 6]. For instance, methylation of histone H3 at lysine 9 (H3K9) is one of the most abundant and stable histone modifications, and is involved in both gene repression and heterochromatin formation. H3K9 can be mono-, di- or trimethylated on H3K9, whereas silent euchromatin regions are enriched for mono- and dimethylated H3K9 [Non-Patent Document 17]. In mammals, heterochromatic regions are highly trimethylated on H3K9, whereas silent euchromatic regions are enriched for mono- and dimethylated H3K9 [Non-Patent Document 17]. H3K9 methylation has been linked to de novo gene silencing and DNA methylation, and it is inherited after mitosis in a manner coupled to DNA methylation.
It has previously been reported that some histone methyltransferases and demethylases are deeply involved in human carcinogenesis [Non-Patent Document 7, 8, 9, 10, and 11]. For instance, SMYD3, PRMT1, PRMT6, SUV420H1 and SUV420H1-2 have been shown to stimulate the proliferation of cells through its enzymatic activity [Patent Document 1, 8, 9, 12, 13, 14, and 18].
SUV39H2, also known as KMT1B [Non-Patent Document 15], is a SET-domain containing histone methyltransferase and is known to methylate the H3K9 lysine residue. Suv39h2, the murine homologue of human SUV39H2, has been isolated and characterized as the second murine Suv39h gene, and demonstrated to share 59% identity with Suv39h1 [Non-Patent Document 16]. The expression of Suv39h2 is restricted to adult testis, and immunolocalization of endogenous Suv39h2 protein reveals enriched distributions at heterochromatin during the first meiotic prophase and in the early stages of sperminogenesis. During mid-pachytene, Suv39h2 specifically accumulates within the chromatin of the silenced sex chromosomes present in the XY body. In addition, the histone methyltransferase activity of Suv39h2 appears to play an important role in regulating higher-order chromatin dynamics during male meiosis [Non-Patent Document 16].